Diffusion lateral transverse

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

Although phospholipids diffuse laterally in the plane of the bilayer and rotate more or less freely about an axis perpendicular to this plane, movements from one side of the bilayer to the other are a different matter. Diffusion across the membrane, a transverse, or flip-flop, motion, requires getting the polar head-group of the phospholipid through the... 

Fig. 13.3. FLAIR (left column) and diffusion-weighted transverse images (right column) in three patients with typical acute new ischemic subcortical lesions. Acute small subcortical lesions occurred in the pons (top row), internal capsule (center row), and lateral to the body of the right lateral ventricle...

Lipids diffuse laterally (horizontally) rapidly but transversely (vertically) slowly. Proteins diffuse laterally. 

Lateral diffusion is in the plane of the membrane, and transverse (flip-flop) diffusion is perpendicular to the membrane (through the membrane). Lateral diffusion (in two dimensions) is fast, and transverse diffusion is slow (or nonexistent) except for gases (C02, NH3) and hydrophobic, uncharged, small molecules (such as cholesterol)... 

The factor Dyo/Dn in Eq. (46) represents the contribution of the jamming effect to the transverse diffusion, and becomes more important at higher polymer concentrations. It is formulated later in this subsection and Sect. 6.3.2. 

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

Integral proteins are usually free to move in the plane of the bilayer by lateral and rotational movement, but are not able to flip from one side of the membrane to the other (transverse movement). Immunofluorescence microscopy may be used to follow the movement of two proteins from different cells following fusion of the cells to form a hybrid heterokaryon. Immediately after fusion the two integral proteins are found segregated at either end of the heterokaryon but with time diffuse to all areas of the cell surface. The distribution of integral proteins within the membrane can be studied by electron microscopy using the freeze-fracture technique in which membranes are fractured along the interface between the inner and outer leaflets. 

When the transverse hopping is diffusive, the effect of tb can only be felt through virtual (perturbative) processes that are faster than the quantum coherence time 1 / 0( ) and in which energy conservation is not required (uncertainty principle). These processes involve pairs of correlated particles in all channels. For instance, a correlated electron-hole pair on one chain can be broken temporarily, one particle hopping to a nearest-neighbor chain (a virtual process) followed a bit later by the other particle, there... 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

Figure 12.31. Lipid Movement in Membranes. Lateral diffusion of lipids is much more rapid than transverse diffusion (flip-flop).

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

One may demand the exact kinetic reason for the occurrence of the multiple humps. Now the nontransient humps seem to coincide with the positions of maximum curvature of the external potential, which at these points changes almost abmptly for larger c from a rather flat to a very steep slope. Thus one may conclude that the random walker, which is driven towards these flanks by the anomalously strong Levy diffusivity, is thwarted, thus the PDF accumulates close to these points. Apart from this rudimentary explanation, we do not yet have a more intuitive argument for the existence of the humps and their bifurcations, we also remark that other systems exist where multimodality occurs, for instance, in the transverse fluctuations of a grafted semiflexible polymer [67]. We will later return to the issue of finite variance in the discussion of the velocity distribution of a Levy flight. 

Reactivity in aggregates may be used to get useful infonnation on mobility in these systems. Vesicles are particularly amenable to these studies because, as mentioned earlier, mobihty in these aggregates is lower than in micelles. For instance, it is estimated that above T, lateral diffusion of the lipids within the plane of the vesicle bilayer is very fast (diffusion coefficient of 10 cm s , in the fluid phase), though three orders of magnitude slower than in an aqueous medium. Accordingly, randomization of a hpid in a leaflet of the bilayer of a 500 A vesicle will occur in milliseconds, whereas the slow transverse (flip-flop) movement from one leaflet to another may take up to several days [1, 7, 60]. 

Biological membranes are fluid in nature. For example, when individual cells with different surface protein markers are fused, the initially separated proteins rapidly mix on the newly formed hybrid (Figure 10.11), This phenomenon is known as lateral diffusion, because molecules move laterally within the plane of the membrane. By contrast, in the much less frequent transverse diffusion, a molecule moves from one side of the lipid bilayer to the other. 

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